1. Field of the Invention
The present invention relates to mobile terminals for mobile satellite services (MSS) communications networks and more particularly to such mobile terminals which are adapted to operate in a net radio communications mode.
2. Description of Prior Art
In U.S. Pat. No. 5,404,375, entitled "Process and Apparatus For Satellite Data Communications" which issued to Brian W. Kroeger et al on Apr. 14, 1995, and assigned to the assignee of the subject invention, there is disclosed a system for transmitting and receiving outbound and inbound data signals through a satellite communications network, the system including a satellite and an end user transceiver for transmitting and receiving the inbound and outbound data signals. The end-user transceiver additionally includes means for transmitting the outbound data signal and for receiving the inbound data signal.
Such systems are also known to include a net radio service capability which comprises the satellite equivalent of terrestrial trunked communications systems where a plurality of designated users of a "net" need to communicate with each other in a closed user group that allows each member of the net to hear what any other user is saying. Each member of the net can also talk when required and thus the system acts like a radio multi-party line.
Presently, satellite systems that cover large geographical areas typically use several satellites that follow different paths at low or medium altitudes so that at least one satellite is at all times covering the desired geographical area. From the standpoint of receiving signals, the low and medium altitude satellites have the advantage of being able to transmit a signal that reaches a mobile terminal unit at the earth's surface with a relatively large amplitude and without appreciable fading. However, such satellite networks are limited in their coverage area per satellite.
It has been recently proposed, to provide a satellite communications network that utilizes a high altitude geosynchronous satellite which is capable of covering an area corresponding to a substantial portion of the earth, for example, North America, so that a total of approximately 6 satellite beams will cover the entire continent from Alaska to Mexico. The satellite for such a network will be approximately 22,600 miles above the equator and will be designed to operate in the L-Band of RF frequencies. For example, the frequency of the signal being transmitted to the satellite will be between 1626 MHz to 1660 MHz; and the frequency of the signal received from the satellite will be between 1525 MHz to 1559 MHz.
Energy travelling this great distance undergoes huge attenuation such that the power flux density incident at the antenna of the mobile unit is approximately 10.sub.-14 watts per square meter. This grossly attenuated signal is further degraded by background noise, and other satellite channel impairments such as Rician fading.
Mobile terminal units capable of receiving signals of this order of amplitude were either limited to paging signals or required extremely sophisticated hardware, which is bulky and heavy and does not lend itself to mobility. Also, the limited functions and expense of such terminal equipment prevent its acceptance among a large segment of potential users.
In light of the foregoing, there is a need for a mobile terminal unit that is not only capable of reliably receiving the attenuated signals, subject to Rician fading, of a high altitude satellite for voice, facsimile, and data communication, but also compact, lightweight, and relatively inexpensive to manufacture.